U.S. patent application number 16/333947 was filed with the patent office on 2019-07-04 for method for producing a flat steel product made of a manganese-containing steel, and such a flat steel product.
This patent application is currently assigned to Salzgitter Flachstahl GmbH. The applicant listed for this patent is Salzgitter Flachstahl GmbH. Invention is credited to THOMAS EVERTZ, KAI KOHLER, MANUEL OTTO, PETER PALZER.
Application Number | 20190203311 16/333947 |
Document ID | / |
Family ID | 59887258 |
Filed Date | 2019-07-04 |
United States Patent
Application |
20190203311 |
Kind Code |
A1 |
PALZER; PETER ; et
al. |
July 4, 2019 |
METHOD FOR PRODUCING A FLAT STEEL PRODUCT MADE OF A
MANGANESE-CONTAINING STEEL, AND SUCH A FLAT STEEL PRODUCT
Abstract
The invention relates to a method for producing a flat steel
product made of a medium manganese steel having a TRIP/TWIP effect.
The aim of the invention is to achieve an improvement in the yield
strength when a sufficient residual deformability of the produced
flat steel product is obtained. This aim is achieved by the
following steps: cold rolling a hot or cold strip, annealing the
cold-rolled hot or cold strip at 500 to 840.degree. C. for 1 minute
to 24 hours, temper rolling or finishing the annealed hot or cold
strip to form a flat steel product having a degree of deformability
between 0.3% and 60%. The invention further relates to a flat steel
product produced according to said method and to a use thereof.
Inventors: |
PALZER; PETER; (Liebenburg,
DE) ; EVERTZ; THOMAS; (Peine, DE) ; OTTO;
MANUEL; (Cremlingen, DE) ; KOHLER; KAI;
(Nordstemmen, DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Salzgitter Flachstahl GmbH |
Salzgitter |
|
DE |
|
|
Assignee: |
Salzgitter Flachstahl GmbH
Salzgitter
DE
|
Family ID: |
59887258 |
Appl. No.: |
16/333947 |
Filed: |
September 13, 2017 |
PCT Filed: |
September 13, 2017 |
PCT NO: |
PCT/EP2017/072994 |
371 Date: |
March 15, 2019 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C22C 38/16 20130101;
C22C 38/18 20130101; C22C 38/38 20130101; C22C 38/24 20130101; C21D
8/0242 20130101; C22C 38/001 20130101; C22C 38/20 20130101; C22C
38/30 20130101; C22C 38/22 20130101; C22C 38/06 20130101; C22C
38/02 20130101; C21D 6/005 20130101; C22C 38/14 20130101; C22C
38/10 20130101; C22C 38/28 20130101; C22C 38/26 20130101; C21D
2201/02 20130101; C21D 2211/008 20130101; C22C 38/12 20130101; C21D
6/007 20130101; C21D 2211/001 20130101; C21D 8/0205 20130101; C22C
38/32 20130101; C21D 8/0236 20130101; C21D 6/008 20130101; C21D
8/0273 20130101; C21D 9/46 20130101; C22C 38/60 20130101; C21D
6/002 20130101; C22C 38/008 20130101; C22C 38/04 20130101; C21D
8/0268 20130101 |
International
Class: |
C21D 8/02 20060101
C21D008/02; C22C 38/60 20060101 C22C038/60; C22C 38/20 20060101
C22C038/20; C22C 38/22 20060101 C22C038/22; C22C 38/24 20060101
C22C038/24; C22C 38/26 20060101 C22C038/26; C22C 38/28 20060101
C22C038/28; C22C 38/30 20060101 C22C038/30; C22C 38/32 20060101
C22C038/32; C22C 38/38 20060101 C22C038/38; C22C 38/00 20060101
C22C038/00; C22C 38/06 20060101 C22C038/06; C22C 38/02 20060101
C22C038/02; C21D 9/46 20060101 C21D009/46; C21D 6/00 20060101
C21D006/00 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 16, 2016 |
DE |
10 2016 117 508.0 |
Claims
1-13. (canceled)
14. A method for producing a flat steel product of medium manganese
steel haying a TRIP/TWIP effect, said method comprising: cold
rolling a hot or cold strip; annealing the cold-rolled hot or cold
strip at a temperature of 500 to 840.degree. C. for 1 min to 24 h;
and temper rolling or skin pass rolling the annealed hot or cold
strip to form a fiat steel product having a degree of deformation
between 0.3% and 60%.
15. The method of claim 14, wherein the annealed hot or cold strip
is temper rolled with a degree of deformation between 10 to
40%.
16. The method of claim 14, wherein the annealed hot or cold strip
is skin pass rolled with a degree of deformation between 0.6 to
2.2%.
17. The method of claim 14, wherein the hot or cold strip is cold
rolled in a first rolling pass at a temperature of the hot or cold
strip of 60.degree. C. to below Ac3, preferably 60.degree. C. to
450.degree. C.
18. The method of claim 17, further comprising intermediately
heating or intermediately cooling the hot or cold strip between
rolling passes following the first rolling pass to a temperature of
60.degree. C. to below Ac3, preferably 60.degree. C. to 450.degree.
C.
19. The method of claim 14, wherein the annealed hot or cold strip
is temper rolled or skin pass rolled at a temperature of 0 to
400.degree. C.
20. The method of claim 14, wherein the annealed hot or cold strip
is temper rolled or skin pass rolled to form the flat steel product
until the flat steel product has a yield strength which is
increased by at least 50 MPa compared with prior to the temper
rolling or skin pass rolling.
21. The method of claim 14, wherein the flat steel product has a
tensile strength of greater than 1300 MPa and an elongation at
fracture A80 of greater than 3%.
22. The method of claim 14, wherein the annealed hot or cold strip
is temper rolled or skin pass rolled to form the flat steel product
until a metastable austenite thereof is partially converted into
deformation twins (TWIP effect) and martensite (TRIP effect),
wherein at least a portion of 3% of the metastable austenite is
converted into martensite and at least a portion of 10% of the
metastable austenite is retained as a face-centred cubic phase.
23. The method of claim 14, wherein the flat steel product
comprises, in wt. %: C: 0.0005 to 0.9, preferably 0.05 to 0.35, Mn:
4 to 12, preferably greater than 5 to less than 10, with the
remainder being iron including unavoidable steel-associated
elements.
24. The method of claim 23, further comprising adding to the flat
steel product by alloying, in wt. %: Al: 0 to 10, preferably 0.05
to 5, particularly preferred greater than 0.5 to 3, Si: 0 to 6,
preferably 0.05 to 3, particularly preferred 0.1 to 1.5, Cr: 0 to
6, preferably 0.1 to 4, particularly preferred greater than 0.5 to
2.5, Nb: 0 to 1, preferably 0.005 to 0.4, particularly preferred
0.01 to 0.1, V: 0 to 1.5, preferably 0.005 to 0.6, particularly
preferred 0.01 to 0.3, Ti: 0 to 1.5, preferably 0.005 to 0.6,
particularly preferred 0.01 to 0.3, Mo: 0 to 3, preferably 0.005 to
1.5, particularly preferred 0.01 to 0.6, Sn: 0 to 0.5, preferably
less than 0.2, particularly preferred less than 0.05, Cu: 0 to 3,
preferably less than 0.5, particularly preferred less than 0.1, W:
0 to 5, preferably 0.01 to 3, particularly preferred 0.2 to 1.5,
Co: 0 to 8, preferably 0.01 to 5, particularly preferred 0.3 to 2,
Zr: 0 to 0.5, preferably 0.005 to 0.3, particularly preferred 0.01
to 0.2, Ta: 0 to 0.5, preferably 0.005 to 0.3, particularly
preferred 0.01 to 0.1, Te: 0 to 0.5, preferably 0.005 to 0.3,
particularly preferred 0.01 to 0.1, B: 0 to 0.15, preferably 0.001
to 0.08, particularly preferred 0.002 to 0.01, P: less than 0.1,
preferably less than 0.04, S: less than 0.1, preferably less than
0.02, N: less than 0.1, preferably less than 0.05.
25. The method of claim 14, further comprising coating the flat
steel product metallically, inorganically or organically.
26. A flat steel product having a degree of deformation between
0.3% and 60% and produced by cold rolling a hot or cold strip,
annealing the cold-rolled hot or cold strip at a temperature of 500
to 840.degree. C. for 1 min to 24 h, and temper rolling or skin
pass rolling the annealed hot or cold strip.
27. The flat steel product of claim 26, wherein the flat steel
product has a tensile strength of greater than 1300 MPa and an
elongation at fracture A80 of greater than 3%.
28. The flat steel product of claim 26, wherein the flat steel
product comprises, in wt. %: C: 0.0005 to 0.9, preferably 0.05 to
0.35, Mn: 4 to 12, preferably greater than 5 to less than 10, with
the remainder being iron including unavoidable steel-associated
elements.
29. The flat steel product of claim 28, further comprising, in wt.
%: Al: 0 to 10, preferably 0.05 to 5, particularly preferred
greater than 0.5 to 3, Si: 0 to 6, preferably 0.05 to 3,
particularly preferred 0.1 to 1.5, Cr: 0 to 6, preferably 0.1 to 4,
particularly preferred greater than 0.5 to 2.5, Nb: 0 to 1,
preferably 0.005 to 0.4, particularly preferred 0.01 to 0.1, V: 0
to 1.5, preferably 0.005 to 0.6, particularly preferred 0.01 to
0.3, Ti: 0 to 1.5, preferably 0.005 to 0.6, particularly preferred
0.01 to 0.3, Mo: 0 to 3, preferably 0.005 to 1.5, particularly
preferred 0.01 to 0.6, Sn: 0 to 0.5, preferably less than 0.2,
particularly preferred less than 0.05, Cu: 0 to 3, preferably less
than 0.5, particularly preferred less than 0.1, W: 0 to 5,
preferably 0.01 to 3, particularly preferred 0.2 to 1.5, Co: 0 to
8, preferably 0.01 to 5, particularly preferred 0.3 to 2, Zr: 0 to
0.5, preferably 0.005 to 0.3, particularly preferred 0.01 to 0.2,
Ta: 0 to 0.5, preferably 0.005 to 0.3, particularly preferred 0.01
to 0.1, Te: 0 to 0.5, preferably 0.005 to 0.3, particularly
preferred 0.01 to 0.1, B: 0 to 0.15, preferably 0.001 to 0.08,
particularly preferred 0.002 to 0.01, P: less than 0.1, preferably
less than 0.04, S: less than 0.1, preferably less than 0.02, N:
less than 0.1, preferably less than 0.05.
30. The flat steel product of claim 26, further comprising a
metallic, inorganic or organic coating.
31. The flat steel product of claim 26, configured for use in the
automotive industry, rail vehicle construction, shipbuilding, plant
design, infrastructure, mining industry, aerospace industry,
household appliance and in a tailored welded blank.
Description
[0001] The invention relates to a method for producing a flat steel
product consisting of a medium manganese steel with a TRIP/TWIP
effect, to a flat steel product produced by this method, and to a
use therefor.
[0002] European patent application EP 2 383 353 A2 discloses a flat
steel product consisting of a manganese steel which has a tensile
strength of 900 to 1500 MPa and consists of the following elements
(contents in weight percent in relation to the steel melt): C: to
0.5; Mn: 4 to 12.0; Si: up to 1.0; Al: up to 3.0; Cr: 0.1 to 4.0;
Cu: up to 4.0; Ni: up to 2.0; N: up to 0.05; P: up to 0.05; S: up
to 0.01, with the remainder being iron and unavoidable impurities.
Optionally, one or more elements from the group "V, Nb, Ti" are
provided, wherein the sum of the contents of these elements is at
most equal to 0.5. This steel is said to be characterised in that
it can be produced in a more cost-effective manner than high
manganese steels and at the same time has high elongation at
fracture values and, associated therewith, a considerably improved
deformability.
[0003] Also, German laid-open document DE 10 2012 013 113 A1
already describes so-called TRIP steels which have a predominantly
ferritic basic microstructure having incorporated residual
austenite which can convert into martensite during deformation
(TRIP effect). Owing to its intense cold-hardening, the TRIP steel
achieves high values for uniform elongation and tensile strength.
TRIP steels are suitable for use inter alia in structural
components, chassis components and crash-relevant components of
vehicles, as sheet metal blanks and as tailored welded blanks.
[0004] German laid-open document DE 10 2015 111 866 A1 discloses a
deformable lightweight steel having a manganese content of 3 to 30
wt. % and TRIP/TWIP properties which has improved material
properties by adding by alloying of up to 0.8 wt. % antimony (Sb)
and a targeted heat treatment at 480 to 770.degree. C. for 1 minute
to 48 hours. In particular, this steel has--in addition to an
improved tensile strength and elongation at fracture, an increased
resistance to hydrogen-induced crack formation and hydrogen
embrittlement.
[0005] German laid-open document DE 10 2005 052 774 A1 discloses a
method for producing hot strips with TRIP and/or TWIP properties
and high tensile strengths. The lightweight steel consisting of the
main elements Fe, Mn, Si and Al is cast into a pre-strip
approximating the final dimensions in protective gas, which
pre-strip subsequently passes through a homogenisation zone. Then,
hot rolling occurs until the predetermined overall degree of
deformation of greater than 70% is achieved. The hot strip is then
annealed in a recrystallising manner prior to cold-forming.
Following this, the finished hot strip is cooled and cold rolled
multiple times, wherein intermediate annealing processes are
performed as required between the individual cold rolling
processes.
[0006] Furthermore, German patent DE 10 2004 054 444 B3 discloses a
method for producing metal components or semi-finished products
with high strength and plasticity by cold-forming of steels. The
cold-forming of the steels is said to lead to hardening by TWIP
(Twinning Induced Plasticity) or SIP (Shearband Induced Plasticity)
effects. The degrees of deformation in the case of full elongation
is in the range of 10 to 70%. Deformation occurs after final-stage
or crystallisation annealing until a strength increase of at least
30% of the starting value is achieved and the remaining tensile
elongation of the metal falls to not lower than 20%. This
deformation process with high elongation is said to be advantageous
in that, despite the high strength values, a plasticity reserve is
retained which allows subsequent final forming into a finished
component by means of conventional forming techniques. The steels
selected for this are characterised by an Mn content in wt. % of 10
to 30. Such high manganese, alloyed steels are more costly than
medium manganese steels owing to the high contents of alloying
elements.
[0007] Proceeding therefrom, the object of the present invention is
to provide a method for producing a flat steel product consisting
of a medium manganese steel, a flat steel product produced by this
method and a use therefor, which objects are characterised by an
improvement in the yield strength whilst obtaining a sufficient
residual deformation capability of the produced flat steel
product.
[0008] This object is achieved by a method for producing a flat
steel product consisting of a medium manganese steel having a
TRIP/TWIP effect having the features of claim 1, a flat steel
product produced by this method having the features of claim 12 and
a use for this flat steel product according to claim 13.
Advantageous embodiments of the invention are described in the
dependent claims.
[0009] In accordance with the invention, by means of a method for
producing a flat steel product consisting of a medium manganese
steel having a TRIP/TWIP effect, comprising the steps of: --cold
rolling a hot or cold strip, --annealing the cold-rolled hot or
cold strip at 500 to 840.degree. C. for 1 min to 24 h, --temper
rolling or skin pass rolling the annealed hot or cold strip to form
a flat steel product with a degree of deformation between 0.3% and
60%, it is achieved that the yield strength of the flat steel
product is increased owing to the temper rolling or skin pass
rolling of the flat steel product. In a conventional manner, the
degree of deformation relates to the thickness direction of the
flat steel product. By way of the increase in the yield strength,
optimised components having a lower sheet thickness can be produced
from this flat steel product. The temper rolling or skin pass
rolling causes a partial conversion of the metastable austenite of
the annealed hot or cold strip into deformation twins (TWIP effect)
and martensite (TRIP effect), wherein at least a portion of 3% of
the austenite has to be converted into martensite and at least a
portion of 10% of the austenite is retained as the face-centred
cubic phase.
[0010] In relation to the temper rolling, provision is preferably
made that the annealed hot or cold strip is temper rolled with a
degree of deformation between 10 to 40%.
[0011] In relation to the skin pass rolling, provision is
preferably made that the annealed hot or cold strip is skin pass
rolled with a degree of deformation between 0.6 to 2.2%.
[0012] Provision is preferably made that the annealed hot or cold
strip is temper rolled or skin pass rolled at a temperature of 0 to
400.degree. C. Deformation twins are hereby formed (TWIP effect)
which increase the yield strength and/or elasticity limit in a
similar manner to the dislocation density of other types of
steel.
[0013] In a preferred embodiment, the annealed hot or cold strip is
temper rolled or skin pass rolled to form a flat steel product
until the flat steel product has a yield strength which is
increased by at least 50 MPa compared with the state prior to the
temper rolling or skin pass rolling.
[0014] In a particularly preferred manner, provision is made that
the flat steel product has a tensile strength of greater than 1300
MPa and an elongation at fracture A80 of greater than 3%.
[0015] In an advantageous embodiment of the method, the hot or cold
strip is cold rolled in a first rolling pass at a temperature of
the hot or cold strip of 60.degree. C. to below Ac3, preferably
60.degree. C. to 450.degree. C. The hot or cold strip is then
optionally intermediately heated or intermediately cooled between
the subsequent rolling passes following the first rolling pass to
temperatures of 60.degree. C. to below Ac3, preferably 60.degree.
C. to 450.degree. C. A reduction in the required deformation forces
is also associated with the increase in the temperature prior to
the first rolling pass. An increase in the residual deformation
capability of the cold-rolled hot or cold strip with tensile
strengths of greater than 800 MPa to 2000 MPa at elongations of
fracture of greater than 3% is also produced in the regions which
are deformed to the greatest extent. The hot or cold strip can be
pre-heated for a coil or wound strip or panel material. By way of
the cold rolling with pre-heating of the hot or cold strip prior to
the first deformation step, conversion of the metastable austenite
into martensite (TRIP effect) is completely or partially suppressed
during the rolling process, wherein deformation twins (TWIP effect)
can form in the austenite. An advantageous reduction in the rolling
forces is hereby achieved, and the overall deformation capability
is increased. By way of the subsequent rolling passes at elevated
temperatures, deformation twins are introduced in a targeted manner
which are then converted into martensite at room temperature and as
a result increase the energy absorption capability and permit a
higher degree of deformation.
[0016] The flat steel product in accordance with the invention is
understood to mean a cold-temper-rolled thick plate, hot strip
and/or cold strip.
[0017] In a particularly preferred manner, provision is made that
the flat steel product is produced with the following chemical
composition (in wt. %) in order to achieve the described
advantages: [0018] C: 0.0005 to 0.9, preferably 0.05 to 0.35 [0019]
Mn: 4 to 12, preferably greater than 5 to less than 10 [0020] with
the remainder being iron including unavoidable steel-associated
elements, [0021] with optional addition by alloying of: [0022] Al:
0 to 10, preferably 0.05 to 5, particularly preferred greater than
0.5 to 3 [0023] Si: 0 to 6, preferably 0.05 to 3, particularly
preferred 0.1 to 1.5 [0024] Cr: 0 to 6, preferably 0.1 to 4,
particularly preferred greater than 0.5 to 2.5 [0025] Nb: 0 to 1,
preferably 0.005 to 0.4, particularly preferred 0.01 to 0.1 [0026]
V: 0 to 1.5, preferably 0.005 to 0.6, particularly preferred 0.01
to 0.3 [0027] Ti: 0 to 1.5, preferably 0.005 to 0.6, particularly
preferred 0.01 to 0.3 [0028] Mo: 0 to 3, preferably 0.005 to 1.5,
particularly preferred 0.01 to 0.6 [0029] Sn: 0 to 0.5, preferably
less than 0.2, particularly preferred less than 0.05 [0030] Cu: 0
to 3, preferably less than 0.5, particularly preferred less than
0.1 [0031] W: 0 to 5, preferably 0.01 to 3, particularly preferred
0.2 to 1.5 [0032] Co: 0 to 8, preferably 0.01 to 5, particularly
preferred 0.3 to 2 [0033] Zr: 0 to 0.5, preferably 0.005 to 0.3,
particularly preferred 0.01 to 0.2 [0034] Ta: 0 to 0.5, preferably
0.005 to 0.3, particularly preferred 0.01 to 0.1 [0035] Te: 0 to
0.5, preferably 0.005 to 0.3, particularly preferred 0.01 to 0.1
[0036] B: 0 to 0.15, preferably 0.001 to 0.08, particularly
preferred 0.002 to 0.01 [0037] P: less than 0.1, preferably less
than 0.04 [0038] S: less than 0.1, preferably less than 0.02 [0039]
N: less than 0.1, preferably less than 0.05.
[0040] This flat steel product consisting of the medium manganese
TRIP (TRansformation Induced Plasticity) and/or TWIP (TWinning
Induced Plasticity) steel has excellent cold-formability and
warm-formability, increased resistance to hydrogen-induced delayed
crack formation (delayed fracture), to hydrogen embrittlement and
to liquid metal embrittlement during welding in the galvanised
state.
[0041] In a conventional manner, the previously described flat
steel product is produced by a production route described
hereinafter: [0042] melting a steel melt with the above-described
chemical composition in a, via the process route, blast furnace
steel plant or electric arc furnace steel plant with optional
vacuum treatment of the melt; [0043] casting the steel melt to form
a pre-strip by means of a horizontal or vertical strip casting
process approximating the final dimensions or casting the steel
melt to form a slab or thin slab by means of a horizontal or
vertical slab or thin slab casting process, [0044] heating the
pre-strip to a rolling temperature of 1050 to 1250.degree. C. or
in-line rolling out of the casting heat (first heat), [0045] hot
rolling the pre-strip or the slab or the thin slab to form a hot
strip having a thickness of 20 to 0.8 mm at a final rolling
temperature of 1050 to 800.degree. C., [0046] reeling the hot strip
at a temperature of more than 100 to 800.degree. C., [0047]
acid-cleaning the hot strip, [0048] annealing the hot strip in a
continuous annealing installation or batch-type--or
discontinuous--annealing installation for an annealing time of 1
min to 24 h and at temperatures of 500.degree. C. to 840.degree.
C., [0049] cold rolling the hot strip at room temperature,
preferably with pre-heating to 60.degree. C. to below Ac3
temperature, preferably 60.degree. C. to 450.degree. C. prior to
the first rolling pass to reduce the rolling forces and form
deformation twins in the austenite and, as required, cooling or
heating between the rolling passes to 60.degree. C. to below the
Ac3 temperature, preferably 60.degree. C. to 450.degree. C., [0050]
annealing the cold-rolled hot or cold strip at 500 to 840.degree.
C. for 1 min to 24 h via continuous or batch-type annealing, [0051]
temper rolling or skin pass rolling the annealed hot or cold strip
to increase the yield strength with smooth or textured rolls (e.g.
with PRETEX texturing), [0052] optionally electrolytically
galvanising or hot-dip galvanising the steel strip or applying
another organic or inorganic coating, [0053] optionally annealing
at 500 to 840.degree. C. for 1 min to 24 h in a continuous
annealing installation or batch-type--or other
discontinuous--annealing installation.
[0054] Typical thickness ranges for the pre-strip are 1 mm to 35 mm
and for slabs and thin slabs they are 35 mm to 450 mm. Provision is
preferably made that the slab or thin slab is hot rolled to form a
hot strip having a thickness of 20 mm to 0.8 mm or the pre-strip,
cast to approximately the final dimensions, is hot rolled to form a
hot strip having a thickness of 8 mm to 0.8 mm. The cold strip has
a thickness of typically less than 3 mm, preferably 0.1 to 1.4
mm.
[0055] In the context of the above method in accordance with the
invention, a pre-strip produced with the two-roller casting process
and approximating the final dimensions and having a thickness of
less than or equal to 3 mm, preferably 1 mm to 3 mm is already
understood to be a hot strip. The pre-strip thus produced as a hot
strip does not have a cast structure owing to the introduced
deformation of the two rollers running in opposite directions. Hot
rolling thus already takes place in-line during the two-roller
casting process which means that separate heating and hot rolling
is not necessary.
[0056] The cold rolling of the hot strip can take place at room
temperature or advantageously at elevated temperature with one
heating process prior to the first rolling pass and/or with heating
processes in a subsequent rolling pass or between several rolling
passes. The cold rolling at elevated temperature is advantageous in
order to reduce the rolling forces and to aid the formation of
deformation twins (TWIP effect). Advantageous temperatures of the
material being rolled prior to the first rolling pass are
60.degree. C. to below Ac3 temperature, preferably 60 to
450.degree. C.
[0057] If the cold rolling is performed in a plurality of rolling
passes, it is advantageous to intermediately heat or cool down the
steel strip between the rolling passes to a temperature of
60.degree. C. to below Ac3 temperature, preferably 60.degree. C. to
450.degree. C. because the TWIP effect is brought to bear in a
particularly advantageous manner in this region. Depending upon the
rolling speed and degree of deformation, intermediate heating, e.g.
at very low degrees of deformation and rolling speeds, and also
additional cooling, caused by heating the material with rapid
rolling and high degrees of deformation, can be performed.
[0058] After cold rolling of the hot strip at room temperature, the
steel strip is to be annealed in a continuous annealing
installation or batch-type--or other discontinuous--annealing
installation advantageously for an annealing time of 1 min to 24 h
and at temperatures of 500 to 840.degree. C., in order to restore
sufficient forming properties. If required in order to achieve
specific material properties, this annealing procedure can also be
performed with the steel strip rolled at elevated temperature.
[0059] After the annealing treatment, the steel strip is
advantageously cooled to a temperature of 250.degree. C. to room
temperature and subsequently, if required, in order to adjust the
required mechanical properties, in the course of ageing treatment,
is reheated to a temperature of 300 to 450.degree. C., is
maintained at this temperature for up to 5 min and subsequently is
cooled to room temperature. The ageing treatment can be performed
advantageously in a continuous annealing installation.
[0060] The flat steel product produced hi this manner can
optionally be electrolytically galvanised or hot-dip galvanised. In
one advantageous development, the steel strip produced in this
manner acquires a coating on an organic or inorganic basis instead
of or after the electrolytic galvanising or hot-dip galvanising.
They can be e.g. organic coatings, synthetic material coatings or
lacquers or other inorganic coatings, such as e.g. iron oxide
layers.
[0061] In accordance with the invention, a use of a component
produced by the previously described method is advantageously
provided in the automotive industry, rail vehicle construction,
shipbuilding, plant design, infrastructure, the aerospace industry,
household appliances and in tailored welded blanks.
[0062] A flat steel product produced by the method in accordance
with the invention advantageously has an elasticity limit Rp0.2 of
300 to 1350 MPa, a tensile strength Rm of 1100 to 2200 MPa and an
elongation at fracture A80 of more than 4 to 41%, wherein high
strengths tend to be associated with lower elongations at fracture
and vice versa: [0063] Rm of over 1100 to 1200 MPa:
Rm.times.A80.gtoreq.25000 up to 45000 [0064] Rm of over 1200 to
1400 MPa: Rm.times.A80.gtoreq.20000 up to 42000 [0065] Rm of over
1400 to 1800 MPa: Rm.times.A80.gtoreq.10000 up to 40000 [0066] Rm
of over 1800 MPa: Rm.times.A80.gtoreq.7200 up to 20000
[0067] The test piece type 2 having an initial measuring length of
A80 was used for the elongation at fracture tests as per DIN 50
125.
[0068] The use of the term "to" in the definition of the content
ranges, such as e.g. 0.01 to 1 wt. %, means that the limit
values--0.01 and 1 in the example--are also included.
[0069] Alloy elements are generally added to the steel in order to
influence specific properties in a targeted manner. An alloy
element can thereby influence different properties in different
steels. The effect and interaction generally depend greatly upon
the quantity, presence of further alloy elements and the solution
state in the material. The correlations are varied and complex. The
effect of the alloy elements in the alloy in accordance with the
invention will be discussed in greater detail hereinafter. The
positive effects of the alloy elements used in accordance with the
invention will be described hereinafter.
[0070] Carbon C: is required to form carbides, stabilises the
austenite and increases the strength. Higher contents of C impair
the welding properties and result in the impairment of the
elongation and toughness properties, for which reason a maximum
content of 0.9 wt. %, preferably 0.35 wt. % is set. In order to
achieve the desired combination of strength and elongation
properties of the material, a minimum addition of 0.0005 wt. %,
preferably 0.05 wt. % is necessary.
[0071] Manganese Mn: stabilises the austenite, increases the
strength and the toughness and renders possible a
deformation-induced martensite formation and/or twinning in the
alloy in accordance with the invention. Contents of less than 4 wt.
% are not sufficient to stabilise the austenite and thus impair the
elongation properties, whereas with contents of 12 wt. % and more
the austenite is stabilised too much and as a result the strength
properties, in particular the 0.2% elasticity limit, are reduced.
For the manganese steel in accordance with the invention having
medium manganese contents, a range of greater than 5 to less than
10 wt. % is preferred.
[0072] Aluminium Al: Al improves the strength and elongation
properties, decreases the relative density and influences the
conversion behaviour of the alloy in accordance with the invention.
Excessively high contents of Al impair the elongation properties.
Higher Al contents also considerably impair the casting behaviour
in the continuous casting process. This produces increased outlay
when casting. High Al contents delay the precipitation of carbides
in the alloy in accordance with the invention. Therefore, an Al
content of 0 to 10 wt. %, preferably 0.05 to 5 wt. %, in a
particularly preferred manner greater than 0.5 to 3 wt. %, is
set.
[0073] Silicon Si: the optional addition of Si in higher contents
impedes the diffusion of carbon, reduces the relative density and
increases the strength and elongation properties and toughness
properties. Furthermore, an improvement in the cold-rollability
could be seen by adding Si by alloying. Higher Si contents result
in embrittlement of the material and negatively influence the hot-
and cold-rollability and the coatability e.g. by galvanising.
Therefore, an Si content of 0 to 6 wt. %, preferably 0.05 to 3 wt.
%, in a particularly preferred manner 0.1 to 1.5 wt. %, is set.
[0074] Chromium Cr: the optional addition of Cr improves the
strength and reduces the rate of corrosion, delays the formation of
ferrite and perlite and forms carbides. Higher contents result in
impairment of the elongation properties. Therefore, a Cr content of
0 to 6 wt. %, preferably 0.1 to 4 wt. %, in a particularly
preferred manner greater than 0.5 to 2.5 wt. %, is set.
[0075] Microalloy elements are generally added only in very small
amounts. In contrast to the alloy elements, they mainly act by
precipitate formation but can also influence the properties in the
dissolved state. Small added amounts of the microalloy elements
already considerably influence the processing properties and final
properties. Particularly in the case of hot-forming, microalloy
elements advantageously influence the recrystallisation behaviour
and effect grain refinement.
[0076] Typical microalloy elements are vanadium, niobium and
titanium. These elements can be dissolved in the iron lattice and
form carbides, nitrides and carbonitrides with carbon and
nitrogen.
[0077] Vanadium V and niobium Nb: these act in a grain-refining
manner in particular by forming carbides, whereby at the same time
the strength, toughness and elongation properties are improved.
Contents of more than 1.5 wt. % or 1 wt. % do not provide any
further advantages. For vanadium and niobium, a minimum content of
0.005 wt. % and a maximum content of 0.6 wt. % or 0.4 wt. % are
optionally preferred, with a minimum content of 0.01 wt. % and a
maximum content of 0.3 wt. % or 0.1 wt. % being particularly
preferred.
[0078] Titanium Ti: acts in a grain-refining manner as a
carbide-forming agent, whereby at the same time the strength,
toughness and elongation properties are improved, and reduces the
inter-crystalline corrosion. Contents of Ti of more than 1.5 wt. %
impair the elongation properties, for which reason a maximum
content of 1.5 wt. %, preferably 0.6 wt. %, in a particularly
preferred manner 0.3 wt. %, is optionally set. Minimum contents of
0.005 wt. %, preferably 0.01 wt. %, can be provided in order to
bind nitrogen and advantageously precipitate Ti.
[0079] Molybdenum Mo: acts as a carbide-forming agent, increases
the strength and increases the resistance to delayed crack
formation and hydrogen embrittlement. High contents of Mo impair
the elongation properties. Therefore, an Mo content of 0 to 3 wt.
%, preferably 0.005 to 1.5 wt. %, in a particularly preferred
manner greater than 0.01 to 0.6 wt. %, is optionally set.
[0080] Tin Sn: tin increases the strength but, similar to copper,
accumulates beneath the scale layer and at the grain boundaries at
higher temperatures. This results, owing to the penetration into
the grain boundaries, in the formation of low-melting phases and,
associated therewith, in cracks in the microstructure and in solder
brittleness, for which reason a maximum content of 0.5 wt. %,
preferably less than 0.2 wt. %, in a particularly preferred manner
less than 0.05 wt. %, is optionally provided.
[0081] Copper Cu: reduces the rate of corrosion and increases the
strength. Contents of above 3 wt. % impair the producibility by
forming low-melting phases during casting and hot rolling, for
which reason a maximum content of 3 wt. %, preferably less than 0.5
wt. %, in a particularly preferred manner less than 0.1 wt. %, is
optionally set.
[0082] Tungsten W: acts as a carbide-forming agent and increases
the strength and heat resistance. Contents of W of more than 5 wt.
% impair the elongation properties, for which reason a maximum
content of 5 wt. % is optionally set. A content of 0.01 wt. % to 3
wt. % is preferred, and 0.2 to 1.5 wt. % is particularly
preferred.
[0083] Cobalt Co: increases the strength of the steel, stabilises
the austenite and improves the heat resistance. Contents of more
than 8 wt. % impair the elongation properties. Therefore, the Co
content is set to at most 8 wt. %, preferably 0.01 to 5 wt. %, in a
particularly preferred manner 0.3 to 2 wt. %.
[0084] Zirconium Zr: acts as a carbide-forming agent and improves
the strength. Contents of Zr of more than 0.5 wt. % impair the
elongation properties. Therefore, a Zr content of 0 to 0.5 wt. %,
preferably 0.005 to 0.3 wt. %, in a particularly preferred manner
0.01 to 0.2 wt. %, is set.
[0085] Tantalum Ta: tantalum acts in a similar manner to niobium as
a carbide-forming agent in a grain-refining manner and thereby
improves the strength, toughness and elongation properties at the
same time. Contents of over 0.5 wt. % do not provide any further
improvement in the properties. Thus, a maximum content of 0.5 wt. %
is optionally set. Preferably, a minimum content of 0.005 and a
maximum content of 0.3 wt. % are set, in which the grain refinement
can advantageously be produced. In order to improve economic
feasibility and to optimise grain refinement, a content of 0.01 wt.
% to 0.1 wt. % is particularly preferably sought.
[0086] Tellurium Te: tellurium improves the corrosion-resistance
and the mechanical properties and machinability. Furthermore, Te
increases the solidity of manganese sulphides (MnS) which, as a
result, is lengthened to a lesser extent in the rolling direction
during hot rolling and cold rolling. Contents above 0.5 wt. %
impair the elongation and toughness properties, for which reason a
maximum content of 0.5 wt. % is set. Optionally, a minimum content
of 0.005 wt. % and a maximum content of 0.3 wt. % are set which
advantageously improve the mechanical properties and increase the
strength of MnS present. Furthermore, a minimum content of 0.01 wt.
% and a maximum content of 0.1 wt. % are preferred which render
possible optimisation of the mechanical properties whilst at the
same time reducing alloy costs.
[0087] Boron B: boron delays the austenite conversion, improves the
hot-forming properties of steels and increases the strength at
ambient temperature. It achieves its effect even with very low
alloy contents. Contents above 0.15 wt. % greatly impair the
elongation and toughness properties, for which reason the maximum
content is set to 0.15 wt. %. Optionally, a minimum content of
0.001 wt. % and a maximum content of 0.08, preferably a minimum
content of 0.002 wt. % and a maximum content of 0.01, is set, in
order to advantageously use the strength-increasing effect of
boron.
[0088] Phosphorus P: is a trace element, it originates
predominately from iron ore and is dissolved in the iron lattice as
a substitution atom. Phosphorous increases the hardness by means of
solid solution hardening and improves the hardenability. However,
attempts are generally made to lower the phosphorous content as
much as possible because inter alia it exhibits a strong tendency
towards segregation owing to its low diffusion rate and greatly
reduces the level of toughness. The attachment of phosphorous to
the grain boundaries can cause cracks along the grain boundaries
during hot rolling. Moreover, phosphorous increases the transition
temperature from tough to brittle behaviour by up to 300.degree. C.
For the aforementioned reasons, the phosphorus content is limited
to values of less than 0.1 wt. %, preferably less than 0.04 wt.
%.
[0089] Sulphur S: like phosphorous, is bound as a trace element in
the iron ore but in particular in the production route via the
blast furnace process in the coke. It is generally not desirable in
steel because it exhibits a tendency towards extensive segregation
and has a greatly embrittling effect, whereby the elongation and
toughness properties are impaired. An attempt is therefore made to
achieve amounts of sulphur in the melt which are as low as possible
(e.g. by deep desulphurisation). For the aforementioned reasons,
the sulphur content is limited to values of less than 0.1 wt. %,
preferably less than 0.02 wt. %.
[0090] Nitrogen N: N is likewise an associated element from steel
production. In the dissolved state, it improves the strength and
toughness properties in steels containing a higher content of
manganese of greater than or equal to 4 wt. % Mn. Lower Mn-alloyed
steels of less than 4 wt. % tend, in the presence of free nitrogen,
to have a strong ageing effect. The nitrogen diffuses even at low
temperatures to dislocations and blocks same. It thus produces an
increase in strength associated with a rapid loss of toughness.
Binding of the nitrogen in the form of nitrides is possible e.g. by
adding titanium or aluminium by alloying, wherein in particular
aluminium nitrides have a negative effect upon the deformation
properties of the alloy in accordance with the invention. For the
aforementioned reasons, the nitrogen content is limited to less
than 0.1 wt. %, preferably less than 0.05 wt. %.
* * * * *